A traditional approach for coupling light from a lamp or other light source into an optical fiber is to concentrate the light at the focal point of an elliptical or parabolic reflector, as generally illustrated in FIG. 1. A typical illuminator employs a lamp as the light source 100, an elliptical reflector 102, a power source 110, and in some cases a motorized color wheel 108. The light from lamp 100 is focused by elliptical reflector 102 onto the input end of an optical fiber 106. Optical fiber 106 is typically a polymeric type fiber optic cable with a relatively large core (between about 1 mm and about 30 mm in diameter) and a jacket (or cladding, typically polytetrafluoroethylene or other polymer or material) having a lower refractive index than that of the core. The recommended service temperature for this type of optical fiber is less than about 80.degree. C., although where the polymeric optical fiber employs a cross-linked polymer the service temperature may be as high as is 120.degree. C. and even as high as 150.degree. C. intermittently. These types of optical fiber are disclosed in detail in U.S. Pat. No. 5,298,327, U.S. Pat. No. 5,579,429, and U.S. Pat. No. 5,067,831. Each of said patents is hereby incorporated by reference as if fully set forth herein.
Unfortunately, typical lamps (such as tungsten halogen lamps and arc lamps) are extended light sources (due to the finite size of the filament or arc, as the case may be), whereas reflectors function most efficiently when the light source is a point source that can be efficiently collected by the reflector and focused to a small spot on the end of the optical fiber. Optical fibers used typically have a core diameter of between about 3 mm and about 15 mm. Filaments and arcs usually cannot be focused efficiently to such a small spot; they are typically focused to larger spots having an intensity distribution peaked in the center and decreasing towards the edges. FIG. 2 illustrates a typical elliptical reflector output from a tungsten halogen lamp having a filament 5 mm long. The focused spot size is approximately 20 mm in diameter, and if such a spot is coupled into a 12 mm core optical fiber a substantial amount of light is lost due to the spot over-filling the core. Also, the intensity distribution over the surface of the optical fiber must be kept to a level sufficiently low so as not to exceed the maximum service temperatures as described above. If the illumination spot from the reflector is too intense, the fiber end may overheat and burn. Therefore, the peak of the intensity distribution shown in FIG. 2 must be kept below a burning threshold, and the remainder of the input area of the optical fiber cannot be illuminated as intensely as the center and the brightness level of the optical fiber output would be correspondingly reduced. The alignment between the lamp, reflector, and optical fiber is of critical importance when using elliptical type reflectors, and when the illuminator requires service or a new lamp is installed re-alignment becomes necessary, since even few millimeters of variation in the filament or arc position results in substantial reduction of optical fiber light output.
Infrared and ultraviolet radiation generated by the lamp must be managed. The ultraviolet radiation can degrade polymeric optical fiber at the input end, thereby substantially reducing light coupling into the fiber optic. Ultraviolet radiation can also photochemically transform certain types of optical fiber into brittle optical fiber, which can be easily broken or cracked. Infrared radiation can cause additional heating at that the input end of the fiber optic, possibly leading to overheating and/or burning of the optical fiber.
The illuminator configuration of FIG. 1 is typically most useful for relatively low power illuminators or applications where relatively low illumination levels are sufficient. Similar illuminator configurations are described in U.S. Pat. No. 4,704,660, U.S. Pat. No. 4,425,599, and U.S. Pat. No. 5,400,225. Each of said patents is hereby incorporated by reference as if fully set forth herein.
Other methods have been used to increase the coupling efficiency of the light from the light source into the optical fiber. One or more lenses located near the input end of the fiber or between the light source and the fiber input end have been used with some success in illuminators having a light source approximating a point source. However, for higher power illuminators where the light source is larger, the light source still cannot be efficiently imaged onto the small core of the fiber input end, and a substantial fraction of the light is lost as described above.
It is therefore desirable to provide an illuminator in which high intensity illumination may be efficiently coupled into and transmitted through an optical fiber. It is therefore desirable to provide an illuminator in which high intensity illumination may be coupled into and transmitted through the optical fiber without overheating and/or burning the optical fiber. It is therefore desirable to provide an illuminator in which ultraviolet and/or infrared radiation are substantially eliminated from the light input into the optical fiber. It is desirable to provide an illuminator wherein various wavelength components may be selected for output from the illuminator.